1. Field of the Invention
The present invention relates to an apparatus and method for forming a silicon-based thin film using a high-frequency plasma CVD process and a semiconductor element comprising the silicon-based thin film.
2. Related Background Art
As a method of forming a silicon-based thin film, the high-frequency plasma process has the advantages of facilitating enlargement of area and formation at low temperatures and increasing process throughput and is thus one of predominant means for mass production.
Considering the production of solar cells as an example of semiconductor elements comprising a silicon-based thin film, when compared with existing energy utilizing fossil fuels, the solar cells using the silicon-based thin films have the advantages of inexhaustible energy sources and clean power generation processes, but it is necessary to further reduce the unit cost per generated power, in order to make them widespread. Technological subjects significant for realizing the cost reduction involve establishment of such techniques as to improve the throughput of the high-frequency plasma CVD process to increase the photoelectric conversion efficiency and to secure a high uniformity when enlarged in area.
In view of the foregoing, there have recently been made various attempts to improve the production techniques of the high-frequency plasma process.
As a forming method for an amorphous silicon semiconductor element, Japanese Patent Publication No. 5-56850 discloses a method wherein the interelectrode distance when an amorphous layer of a large film thickness is formed is set smaller than the interelectrode distance when an amorphous layer of a small film thickness, and the substrate temperature and the vacuum degree are set higher.
Japanese Patent Application Laid-Open No. 2000-252484 discloses a method for manufacturing an amorphous silicon-based thin film photoelectric conversion device having a stack structure of pin, wherein at least one layer is formed under specific conditions, such as making the interelectrode distance 8 mm or more and 15 mm or less.
In a forming method for a silicon-based thin film on a substrate using a high-frequency plasma CVD method under the conditions of specific interelectrode distances is disclosed in the above-described prior art. However, formation of a silicon-based thin film using a plurality of frequencies depending on a silicon-based thin film to be formed, as well as the relationship between the frequencies and the interelectrode distance, and the relationship with the conductivity type of the silicon-based thin film to be formed at that time are not disclosed.